175 hp Compressor Tripping When Switched On to DG Rated 600kva

The 600 kva genset ought to be able to take on this 175 hp load without trouble. If the voltage reading is normal before putting on the compressor, I don't think field flashing will help. I would look at the compressor controls to be sure that the compressor is unloaded on start-up. (Unloading solenoids in case of a recip compressor, capacity slide valve in case of a screw compressor.) The fuel supply to the genset could also be an issue.

it is a screw type compressor.so what are the possible alternatives?

It is mechanical problem. Any a/c technician can find out the problem with screw compressor.

If the compressor is tripping the generator has nothing to do with it. Is it the thermal overload or the instantaneous over current or the under voltage protection which is tripping? Are the process procedures being followed as before?

there is nothing wrong with any protection device.there is a sudden fluctuation in the vtg as the compressor is switched on.why so?

Check voltage at Comp. feeder, if low, correct DG Voltage.

Reduced voltage at motor terminal shall increase starting time of comp motor. Some time tripping of motor.

"there is nothing wrong with any protection device.there is a sudden fluctuation in the vtg as the compressor is switched on.why so?"

How have you determined there is nothing wrong with any protective device? Why have you not told us what the actual trip cause is? How are you defining "fluctuation"? Some disturbance is to be expected. HOW MUCH is an important distinction. Is the compressor motor running when you transfer from the utility power to the DG? Was that the case when it was not tripping in the past?

You are being vague and inaccurate, something I would not expect from an engineer. Is there a reason why you repeatedly ask for help and yet provide insufficient information?

we have already cross checked all the protection devicess.actual trip cause is thus far uncertain.at the moment,just trying to speculate and find out a solution.there is huge dip in voltage (~20v).we have two transformers 1000 kva and 1500 kva.the one which is having the compressor load is showing 395 v,other one is showing 415 v.it was not tripping in the past.it happened only in past few days when there were frequent power interruptions. if you require any other information, do let me know.

Clarity is required on your posting.

1) when the Compressor is started by connecting it to 1000 kVA transformer source-

a) Does it start?

b) if it starts what is the starting time and what is the voltage dip at the motor (compressor ) terminal

c) what is the starting current?

Simalarly note down the detalis when the compressor is connected to 1500 kva transformer source and 600 kVA DG source?

Please also find out the preloading on DG set when you try to connect the compreesor on to DG set source?

Please also inform the type and make of motor protection relay and its settings fitted on to the compressor motor circuit

When you furnish the above information it may be possible to give suitable remedy.

First we have to ascertain whether the Compressor is tripping due to-

a) Undervoltage

b) over current

c) Long starting time

You have indicated that the compressor works (starts)some times and some times it does not work (start)

I have a feeling that preloading on the DG sets will matter in this case.

On 600 kVA DG set ,for 175 hp compressor to start , there should not any preloading beyond 200~250 kVA as a rule of thumb, but actuals have to be workedout based on motor and alternator design parameters and characterstics

My learned friends wrote a number of causes, to add with, I will suggest you to check the Automatic Voltage Regulator of Generator.

Real generators cannot supply many times rated current at rated voltage for long. Have you been able to note the current and voltage after the compressor is energised?? A likely volt-amp draw for the "175 kVA" compressor at RATED voltage is twice the generator rating. Additionally, these VA have a low lagging power factor, which is "worst case" for voltage dip. Supplying over rated current at near normal voltage and low power factor requires well over normal excitation, which cannot be supplied for long because the field windings will overheat. Even though DG set AVRs/generators can supply, say, 3 times rated current at 80% volts, there is an initial dip because the field inductance sets a limit to how fast the excitation can respond - after some seconds the AVR should start cutting the over-excitation to protect the field. I give all preceding "for information". As other posts write, it is far more likely to be trouble with the compressor. I believe compressors are well known for being far more difficult to start when they have recently been running, they can even trip motors on mains supplies - was this the case?? As you will soon learn as a new engineer, people who are not "on-site" are quick to blame someone else's equipment to avoid having to make a visit (or admit they have sold an equipment knowing they can over-charge for modifications to meet the circumstances once it is on site). I have often had to make extensive tests to prove "it cannot be us..." before associated, "off-site", equipment is checked and found to be at fault (or someone did not do a proper "removal from service" and left an inter-tripping circuit active on their side).

In an earlier reply I was imagining that the compressor is the only load. Is it, or are other loads already on the genset when trying to start the compressor? It is best to start the largest load first and save the smallest for last.

Most refrigeration screw compressors use a slide valve for capacity control (unloading), and the compressor control scheme makes the slide valve go to fully unloaded before letting the compressor start. You should verify that this is happening.

What are the alarm/indications in the engine/panel when it trips?.

I'd be curious to know what can feed what in this whole system, and whether the transformers have adjustable taps.

A dip of 20 volts is only about 5%, so I don't yet see how that is the problem.

If possible, try to check the compressor amperage as it starts, and listen for "straining" sounds while starting.

What kind of starter is being used?

I think that engaging in endless speculation about an unknown, although kind of fun. is an exercise in futility.

There is no point in proceeding until you determine WHAT is tripping! Breaker? Protective Relay? Contactor dropping out? VFD? Soft Starter?

I'm invoking Rule 75 here.

Not again.

'Rule 75'..........

Of course there is! Field flushing indicates problem of magnetic field that induces power in the stator of geny. This could lead to total burning of the geny's wiring. Just follow instruction of the generator. It might be that diodes are blown in the field excitation circuits.

This sounds to me like a problem with the unloader valve. If the unloader valve is defective, the compressor will be trying to start against accumulated air under pressure in the line from the compressor to the reservoir. Unloader valves can be either all mechanical type or electrically operated, but release air under pressure between the compressor and reservoir after compressor shutdown to prevent the compressor from trying to start under excessive load.

Dear Mr. Samarpit,

So many replies are seen here. What comes to my mind is as follows.

1. What is the type of starter you are using. Is it slip-ring type, or stat-delta type or direct on-line type.?

2. Before you switch over to the DG set what is the load on the DG set.?

3. How much capacitor you have for this motor. ?

4. What is the time taken to come to full speed.?

5. What is the Gd^2 value of the compressor.? Does the motor Gd^2 value is more than the Gd^2 value of the compressor.?

The starting current or kick load may go up to 6 times the full load current. The full load current of your motor will be in the order of 245 Amps. For 600 KVA set the full load current will be in the order of 875 Amps. The instantaneous kick load will be in the range of 1225 to 1470 Amps if the starter is star-deta type. ( i.e. 5 to 6 times range) and generator amps is only to the extent of 65% (approx.) and hence tripping is occuring.

If the starter is of slip-ring type, the starting current will be in the order of 785 Amps. very near to the full load current of the alternator. If the DG set already have load, then the kick load/starting current issue becomes intensive the voltage drop at the time of starting will be much higher than the permitted range.

As regards the starting time - more the duration, for reaching full speed, longer the value of current higher than the full load current and hence the thermal relay will trip the motor. The Gd^2 value for the motor plays a vital role for this issue, and it should be more than the compressor Gd^2 value. If not the starting time will be more and takes more time for the current to come to full load current level, by that time the thermal relay will trip the supply.

Hence tripping problem is existing on different counts.

Pl. check for the above and post the information how the problem is solved. This is very essential because many CR4 Members come forward with suggestions - it is natural that the Members will be anxious to know as what was the real issue and whose advise helped to solve the issue.

DHAYANANDHAN.S